It has heretofore been known that a vinyl compound such as styrene, acrylic acid, methacrylic acid, an acrylic ester, a methacrylic ester and acrylonitrile has the property of liability to polymerization by light, heat or the like. In the production process of the above-mentioned vinyl compound, various distillation operations are put into practice for the purpose of separating, recovering, concentrating or purifying the objective vinyl compound. However, the aforesaid vinyl compound is apt to bring about an unfavorable situation such as various troubles in the distillation step which finally make it impossible to proceed with a long-term stable continuous operation, since as mentioned above, the compound is liable to polymerization by light, heat or the like to form a polymer-like substance.
In order to avoid such a situation, there has heretofore been adopted a process in which the distillation operations are put into practice in the presence of a polymerization inhibitor, which is exemplified by hydroquinone, methoquinone (p-methoxyphenol), p-tert-butyl catechol, tert-butylhydroquinone and phenothiazine. Nevertheless, the actual situation is that any of the above-mentioned polymerization inhibitors does not necessarily exert sufficient effect on acrylic acid, methacrylic acid and the like because of their extreme liability to polymerization.
Aside from the foregoing, it is known that cupric dibutyldithiocarbamate is a substance which is capable of extremely effectively preventing the polymerization in liquid phase of acrylic acid, methacrylic acid, etc., but it has been difficult to employ the substance in an industrial manufacturing plant because of its fatal disadvantage that it corrodes an equipment made of, for example, SUS 316.
On the other hand, there is proposed a method in which a manganate is employed as a corrosion inhibitor in combination with a metallic salt of dithiocarbamic acid. (Refer to Japanese Patent Application Laid-Open No. 51403/1993). However, the aforesaid method is not satisfactory because of its insufficient corrosion inhibition effect.